1. Field of the Invention
This invention relates to a method of and apparatus for analysing the composition of substances and, more particularly, to a method and apparatus which correct for measurement drift, which occurs over time, in analytical apparatus or instruments, such as mass spectrometers.
2. State of the Art
There is a continuing need for rapid and precise analyses of the chemical or biochemical composition of biological and microbiological systems, both within biotechnology and for the purposes of identifying potentially pathogenic organisms. Pyrolysis mass spectrometry is an instrument-based technique which is currently used to perform such analyses. Pyrolysis is the thermal degradation of complex material in an inert atmosphere or a vacuum, which causes molecules to cleave at their weakest points to produce smaller, volatile fragments. For example, one version of this technique is known as Curie-point pyrolysis which involves drying a sample of the complex material to be analysed onto a metal and then heating the metal to its Curie point over a short space of time (typically 0.5 seconds). The degradation products are then separated and counted by a mass spectrometer in order to produce a pyrolysis mass spectrum which can be used as a `chemical profile` or fingerprint of the complex material undergoing analysis. Pyrolysis mass spectrometry involves minimum sample preparation and enables materials to be analysed directly, i.e. without using a reagent. It is also rapid, quantitative, relatively cost efficient and can be conveniently automated. Pyrolysis mass spectrometry is a well established method within biology and microbiology for the differentiation and identification of groups of bacteria, fungi and yeasts, and has also been applied to the authentication of foodstuffs, because it is a highly discriminatory method of analysis which may be used on any organic material.
In recent years, pyrolysis mass spectrometry techniques have been expanded in order to perform quantitative analysis of the chemical constituents of microbial and other samples, by means of supervised learning methods using artificial neural networks. Multivariate linear regression techniques such as partial least squares (PLS) regression and principal components regression (PCR) have been found to provide an effective method of following the production of indole in a number of strains of E. coli grown on media incorporating amounts of tryptophan, quantifying the chemical or biochemical constituents of complex biochemical binary mixtures of proteins and nucleic acids in glycogen, and measuring the concentrations of tertiary mixtures of cells of the bacteria Bacillus subtilis, Escherichia coli and Staphylococcus aureus. Such techniques, also known as chemometric techniques, have also been used for biotechnological purposes in order to perform quantitative analysis of recombinant cytochrome b.sub.5 expression in E. coli, and in order to effect rapid screening of high-level production of desired substances in fermentor broths. Pyrolysis mass spectrometry techniques combined with artificial neural networks have also been exploited for rapid and accurate assessment of the presence of lower-grade seed oils as adulterants in extra virgin olive oils, and for effecting rapid identification of strains of Eubacterium, Mycobacterium, Propionibacterium spp., and Streptomyces. The use of supervised learning techniques to identify and analyse samples from their pyrolysis mass spectra is advantageous because it eliminates the necessity for interpretation of complex principal components analyses and cononical variates analyses plots employed in the past: the identities of the components in a sample are binary-encoded at the output layer of the neural network such that the results may be easily read.
However, pyrolysis mass spectrometry techniques using neural network analysis are generally limited to short-term identification of components wherein all microorganisms are analysed in a single batch. This is due to the uncertainty of long-term reproduction of the pyrolysis mass spectrometry system. It has been found that sets of data obtained at the beginning and end of a relatively long time period, for example 14 months, cannot be directly compared due to long-term ion source ageing, i.e. over long periods of extended use, intractible organic debris collects around the ion source, which alters the transmissivity of ions and thus causes mass spectral drift.
In order to compensate or correct for mass spectral drift, it is necessary to tune the mass spectrometry instrument. Such tuning is typically achieved using the volatile standard perfluorokerosene and is known in the art. However, this procedure does not compensate for all of the instrumental drift.